Air conditioning is a mystery to most. Here’s how to revive a system and keep it working at optimum efficiency

Though first fitted in the USA in the 1930s, air conditioning was only seen on expensive luxury cars in most of the world until the 1980s. Driving conditions in major markets like Japan and the United States ensured that, once the cost came down, it would be more widely fitted, and now the majority of new cars are equipped with air conditioning.

On classics air conditioning remains a rarity yet, provided it’s in good working order, it can be immensely useful.

A popular misconception is that air conditioning on a classic car is expensive or, worse, impossible to repair. ‘Not at all,’ says James McClean of Motor Climate Services. ‘Older cars generally use universal components which are still widely available. Even Mercedes and BMW’s own systems are repairable and we remanufacture compressors for many classic cars for around $400.’Another popular misconception is that air conditioning should only be used with unheated air. In fact, when used with warm air it is stunningly effective for demisting.

The third popular misconception is that it’s a ‘fit-and-forget’ accessory. In fact it’s vital to run aircon regularly – at least once a month and ideally once a week – and to have it serviced every 30,000 miles or three years. Every system has a ‘filter drier’ which should be changed at this interval; not to do so is like running your engine for ever without changing the oil filter. At the same interval, the gas should be recharged. Even a good system loses 15% of its volume each year by seepage, so recharging really does improve it.

The difference is more noticeable in really hot weather: working properly, aircon reduces cabin temperature to a third of ambient temperature, but on a warm day it may feel cool even when working at low efficiency.

Failing to service a system, particularly one that has not been used regularly, is asking for trouble: ‘Moisture and acid builds up inside if the system is not used,’ explains McClean. ‘It causes surface rust inside and, when you operate the system, the rust gets carried around like a grinding paste. If you don’t change the filter it will eventually wreck the compressor.’

Types of gas usedVehicle air conditioning systems have used two different gases over the years: R12 until 1993-’94, then on later systems R134a. CO2 may take over in future, but requires different piping and seals so will not be compatible with older systems.

R12 is a CFC greenhouse gas, harmful to the environment, and its use has been banned since January 2000. But there are still plenty of cars running around with R12 in their air conditioning systems. As recharging is illegal, this must be professionally collected and the system converted to R134a – a process that costs less than $200 so is not at all expensive, but one that only larger aircon specialists can carry out properly. As time goes on and fewer R12 systems are left, it’s likely to become harder to get this done, so it’s worth converting now.

R134a runs at higher pressure than R12, so a proportionately lower volume is required to get the same result – and it also makes old rubbers go hard, so specialists first add a special oil additive which swells the seals.

Kits are sold in some accessory shops and over the internet that claim to allow anyone to recharge their air conditioning system at home, or even convert from R12 to a replacement gas. All specialists strongly advise against using these, for several reasons. To use them, you would have to vent any remaining R12 to atmosphere, which is illegal. Aircon systems require a measured amount of gas – too much or too little and they won’t work properly – but the kits give no way of monitoring this. The gases provided are not suitable for vehicle systems and are likely to escape within a short time.

If you’re not sure which gas your system has, or if you want to check that a potential purchase really has been converted to R134a as claimed, there is a simple check. All systems have service ports for recharging: on R12 systems, they have an external screw thread, whereas R134a systems use a quick release push-on ring.

The basic componentsAll aircon systems incorporate five major elements. The Compressor receives the refrigerant as a low-pressure gas from the Evaporator and increases its pressure and temperature so it becomes a high-pressure gas. The Compressor is mounted on the engine and is driven by belt from the crankshaft pulley. The most common reasons for failure are lack of oil, pulley bearing wear, electromagnetic coil burn-out and lack of servicing.

From the Compressor, the gas goes to the Condenser. Here the refrigerant gives up its heat as cold air is passed across its fins and tubes by ram effect or by an extra fan. The refrigerant becomes a high-pressure liquid which is forced out to the Receiver Drier.As the Condenser is usually at the front of the car, it can suffer damage and corrosion.

The Filter Drier/Receiver Drier/Accumulator is responsible for removing moisture from the air conditioning system by means of a bag of desiccant which absorbs small quantities of moisture. This should be changed at least every three years or when the system is repaired.

Next is the Expansion Device, which may be a brass internally or externally equalised valve, a block-type valve or an orifice tube. Its inlet and outlet separate the high pressure side of the system from the low side. The valve allows only a small amount of refrigerant to pass through to the Evaporator.

As soon as the liquid pressure drops on passing through the Expansion Device, the refrigerant begins to boil (R134a boils at -26˚ C). The Evaporator absorbs the heat of the air passing over its tubes and fins and as a result the air is cooled before being blown into the car. The Compressor takes the low-pressure vapour from the Evaporator and the cycle starts again.